X-ray screening apparatus



Sept 17, 1946. N, G. scHNANDER 2,407,938

X-RAY SCREENING APPARATUS Filed Nov. 24, 1943 2 Sheets-Sheet 1 NVENTOQ NUS Geox-35cm: n emclvx Sept 17, 1946 N. G. scH'NANDl-:R

X-RY SCREENING APPARATUS 2 sheets-sheet 2 Filed Nov. 24, 1943 u vvvv l.

Nils Georg Sth'nonder h ATTORN EY Patented Sept. 17, 1946 UNITED STATES PATENTOFFI'CE X-RAY SCREENING APPARATUS Nils Georg Schnander, Stockholm, Sweden Application November 24, 1943, Serial No. 511,503 In Sweden March 9,y 1943 (CI. Z50- 62) 7 Claims.

The present invention relates toX-ray apparatus for screening or shutting out undesirable secondary radiation, when for example X-ray radiographs or photographs are being taken.

When a body is being penetrated by X-rays, secondary X-rays are generated in the body and which secondary X-rays scatter in all directions, thus causing an impairment of the contrast of the picture to be obtained by the primary X-rays. Said impairing action of the secondary X-rays can be lessened by introducing between the object to be X-rayed and the photographic plate or iilrn or the uoroscopic plate a screen consisting of ribs, strips or leaves of a material substantially impervious to X-rays, for example lead, which ribs, strips or leaves are placed on edge, and thus to a great extent absorb the secondary rays, while the primary rays can pass through spaces or material transparent to said rays between the said ribs, strips or leaves.

Such screening devices in their simplest form are provided with plane leaves standing on edge at right angles to the base of the screen. This device, however, causes the disadvantage that the primary rays will considerably be shut out at the edges of the screen, due to the fact that the direction of the rays there forms an angle to the surface of the leaves. In order to lessen this disadvantage several remedies have been proposed. Thus somebody has proposed to bend the entire screen in concave shape, for example in the form of a spherical surface, whereby the X-ray focus is located in the center of the sphere.

'I'hese devices, however, have several disadvantages. Thus the sharpness of the picture is successively lessened towards the edges due to the broader shadows of the leaves at said edges. Furthermore the shape of the screen is impracticable, as the object table for practical purposes should be plane. They are besides expensive to make.

It-has furthermore been proposed, while keep` ing the screen plane, to let the planes of the individual vleaves pass through the X-ray focus.

' It has previously been proposed by myself to use the simplest type of screen using thin plane or-curved leaves at right angles to the plane of the screen, whereby the thickness of the screen was decreased towards the edge of the screen, parting from a place, where the primary rays are parallel to the plane or to the generatrix of the surface of the leaves. The advantage of this arrangement is clear, because due to the fact that the height of the leaves decreases when the angle of the 'primary rays to the leaves increases, the screening effect of the leaves to the Primary rays 2 will be less. The carrying principle of' such a construction was such that the shadow of each leaf of said screen should be the same throughout the screen. However, the disadvantage of such a screen only solves the problem, when the X-rays focusV is located at a relatively large dis-V tance from the screen. If said X-ray focus is moved nearer to the screen, the shadows of the leaves towards the edges will be greater than the shadows near to the center. In other words the picture will be lighter or less distinct towards the edges.

The present invention aims to create an mproved screening device eliminating the disadvantage of all `known screening devices.

The present invention thus relates to an X-ray apparatus comprising an X-ray tube, a screen for reducing secondary X-rayA radiation on X-ray uoroscopie or photographic plates or lms and interposed between the object to be X-rayed and the said plate or nlm, said screen comprising on edge standing leaves of material substantially impervious to X-rays, said leaves being alternately interspaced with material substantially trans` parent to X-rays, and means for moving the X-ray tube to vary the distance between the focus of said tube and the said screen, plate or iilm, and the invention is principally characterised by this, that the leaves are convergently directed towards a point located about half way between the extreme positions of said variable focus, whereby the height of said on `edge standing leaves have been lowered towards the edges of said screen, in order thus to obtain substantially the same width of leaf-shadows and sharpness of picture over the entire surface of said plate or iilm at varying distances between the said X-ray focus and the said screen, plate or lm.

The `invention is thus characterised by the two facts that the leaves are inclinedto serve an X-ray focus at a medium distance, for example cm., said medium distance being located between a maximum and a. minimum distance of, for example, and 60 cm. respectively, and that the heights of the leaves decrease towards the edges of the screen, in order to obtain substantially the same width of shadow and sharpness of picture over the entire surface of the screen, even if the distance joftheQX-ray focus` to the screen should considerably vary.

Due to the fact that the leaves are inclined,

land thattheir height decrease when the angle of vthe primary rays to the respective planes increase, the screening eiect of the leaves for the primary rays will lessen, even if the X-ray focus varies within considerable limits. On the same time the screening effect of the secondary rays respect to the source of rays and that it is sym.- metrical itself. rlihe invention, however, comprises also unsymmetrical screens.

emission, and on the same time will permit a variable X-ray focus distance, is easily understood. A variable X-ray focus is an indispensable requirement to enable an operator efficiently to scrutinize, objects of varying size and form, in order to obtain the best results.

One surface of the screen is preferably plane, while the opposite surface may be shaped in various different ways. For instance, the second surface I4 may rise from its peripheral edge to the center of the screen at a constant rate to give 4to the screen the shape of a prism of substantially triangular cross-section (Fig. 1). If desired, the

` screening device is symmetrically located 'with It is not necessary gradually todecrease the height over the entire width of the screen. the primary rays can be permitted a certain angle of .entry towards the leaves before the shadowing effect ofthe screen is noticeable, a certain part at the center ofthe screen can be retained plane and only one or more of the sidev surfaces be inclined'towards, the edge over a plane, concave" or a convex surface, depending upon the mean distance of the focus, for which the screen should function. Y

The invention will be further understood in connection Ywith the accompanying drawings, of which:

Fig. l illustrates diagrammaticallythe principle of the invention, one embodiment of the screen being shown'in cross-section.

Fig. 2 Vshows in cross-section another embodiment of the invention with two combined screening devices.

, Figs. 3, 4 and5 are cross-sections vof three further screen modifications, respectively.

The leaves I are made of metal of preferably high specific gravity, for example-lead, and located at from each other equal distances 2. The spaces 2 between the leaves can\consi st of strips or 4laminae of light metal, paper, Bakelite or similar material that will allow X-rays to pass through same. a

vThe leaves I are substantially directed towards a focus situated at an average from the screen, plate or lm, and they can consist of straight leaves located in planes that pass through said average focus or they can be curved and form part or parts of concentric or spirally shaped sur- Vfaces converging towards said focus.

The X-ray tube `3 with its focus is located at the average distance fm from the back part of the screen, while the do-tted .positions 4 and 5 indicate the maximum 'and minimum distances for the same tube.' The translucent body '6 is located between the X-ray focus and the screen.

Upon moving the X-ray tube to the position 4, the shadow ofthe lead leavesIwill be of about the same size l, '8, over the entire surface of the screen. The secondary rays II from the 'body 6 will entirely be prevented bythe leaves to pass, while the vspaces vin front of the place of secondary emission will allow only a small part I0 of the entire secondary emission to pass.

Being known that 80 to 90% of the blacking of a lm originates from secondary emission of the irradiated or translucent 'body at, for example a voltage of 80 kv. 'impressed onthe X-ray tube, and that this secondary emission will rapidly increase atincreased voltage, the importance of aneflicient screening device, that will simultae l neously shut out; the greater part of the secondary central part of this second screen surface I3 may lbe made plane parallel to the rst surface, so

that the screen assumes the shape of a prism of regular parallel trapezium cross-section (Fig. 3), the Vtop surface parallel to the bottom surface in the embodiment shown comprising about 1/3 of the whole surface of the prism. Alternatively, the second screen surface I5 may be made convex to give to the screen the shape .of a planoconv-.ex lens (Fig. e), or the height of the second surf-,ace I2 may rise from its peripheral edge'towards the center of the screen at agradually. increasing-rate (Fig, 5)'. These alternatives could lead .to a great number of embodiments in the form .of lensrespectively prism-shaped bodies, which all have the advantage of the nearly .complete elimination of the-bad eifects'of vthe secondary ray emission. At the same time the invention offers a simple way of manufacturing and. of fmounting, due .to the substantially plane shape of the'screen, which permits `a variable X-ray focus for practical purposes within amplelimits.

At a high voltage impressed on the X-ray tubeI two screens with straight leavesplacedone Von the top of the other can be used, whereby .the leaves of one screen .are located at substantially right angles to those of the other. v

In this way a double screen is obtained,` which has the advantage that theeffect of the increased secondary emission on the film in :the direction of the straight leaves is considerably limi-ted, and that .a fine screen with -mesh similar to .that-of prints .on paper is obtained.

When double screens are used the spaces be-l tween the. leaves .can `be made somewhat .greater than in single screens in orden-tolet through a greater amount of primary rays onthe same .time that the effects of the secondary emission is lime ited in all directions. In making the spaces tbetween the lead leaves greater, the costof production of each of the two .screens will also =be cheaper.

Fig. 2 shows Van embodiment of the invention, in which two prisms or conically shapedscreens I6 and I7 are shown. In the shown embodiment the truncated vprisms or cones are `placed `with their bases facing each other, whereby the-.leaves of the screen I6 consist'of straight surfaces-the prolonged planes of which would -pass through the average Xfray focus,l while similar leavescf the screen Il preferably and substantiallyare located at right anglesrto Ythe leaves of the screen I6, and also so directed that .their prolonged planes would pass through thesame Xfray focus.

lInstead ofthe screening truncated prisms have ing been placed with their bases Afacing each other, as shown on the drawings, they'ofcourse 5 square, rectangular, rhombic, rhomboidal or trapezoidal mesh.

By making a screen according to this invention with a relatively fine mesh the screen or screens can with advantage be stationary.

I claim:

1. An X-ray apparatus comprising an X-ray tube, a screen for reducing secondary X-ray radiation on X-ray iiuoroscopic or photographic plates or films and interposed between the object to be X-rayed and the said plate or lm, said screen comprising on edge standing leaves of material substantially impervious to X-rays, said leaves being alternately interspaced with material substantially transparent to X-rays, and means for moving the X-ray tube to vary the distance between the focus of said tube and the said screen, plate or film, in which the leaves are convergently directed towards a point located about half way between the extreme positions of said variable focus in which the height of said leaves are lower near the edges of said screen than near the center of the same, in order thus to obtain substantially the same width of shadows and sharpness of picture over the entire surface olf said plate or film at varying distances between the said X-ray focus and the said screen, plate or lm.

2. The X-ray apparatus as claimed in claim 1, in which the screen has the shape of a prism of regular parallel trapezium-formed cross section.

3. The X-ray apparatus as claimed in claim 1, in which the screen has the shape of substantially a prism of triangular-formed cross section.

4. The X-ray apparatus as claimed in claim 1, in which the screen has the shape of a prism of regular parallel trapezium cross section, the top surface parallel to the bottom surface comprising about 1/3 of the whole surface of the prism,

5. In combination with an X-ray apparatus comprising an X-ray tube, two screens for reducing secondary X-ray radiation on X-ray uoroscopic or photographic plates or films and interposed between the object to be X-rayed and the said plate or film, each screen comprising on edge standing leaves of a material substantially impervious to X-rays, said leaves being alternately interspaced with a material substantially transparent to X-rays, and means for moving the X-ray tube to vary the distance between the focus of said tube and the said screen, plate or lm, said leaves being convergently directed towards a point located about half way between the extreme positions of said variable focus and the heights of said leaves being lower near the edges of said screen than near the center of same, the leaves of one screen being placed at an angle to those of the other screen.

6. The combination as claimed in claim 5, in which the surface of each screen is flat on one side and sloping on the other side, the flat surfaces of both screens being adjacent each other and the leaves of one screen being located at about right angles to those of the other screen.

7. The X-ray apparatus, as claimed in claim 1, in which the screen has a plane surface and, opposite said plane surface, a surface the height of which rises from its peripheral edge to the center oi the screen at a gradually increasing rate.

NILs GEORG SCHNANDER. 

